1. Field of the Invention
This invention relates to the treatment of hard waters and more particularly to the softening of such waters by the application of heat to remove scale-forming constituents.
2. Description of the Prior Art
In many industrial applications, it is desirable to soften hard water in order to substantially decrease the concentration of divalent metal ions which form insoluble precipitates, commonly referred to as "scale". For example, it is a conventional practice to treat boiler feed waters in order to avoid the deposition of scale detrimental to the operation of the boiler within pipes, boiler tubes, and other locations. Such deposits are commonly characterized as "soft scale"--calcium carbonate and magnesium hydroxide; and "hard scale"--calcium sulfate, calcium and magnesium silicates, and silica.
Various techniques are employed to soften boiler feed waters and other waters used in industrial applications. For example, a commonly used water treatment process involves lime softening, followed by filtration and ion exchange polishing. In this procedure suitable chemicals, e.g. lime and soda ash, may be added to hard water to produce insoluble precipitates such as calcium carbonate and magnesium hydroxide. The water is then passed through a filter to an ion exchange resin where substantially all of the remaining divalent metal ions are exchanged with sodium from the resin.
Other processes involve the application of heat to the hard water in order to enable or accelerate the chemical reactions leading to the precipitation of the scale forming divalent metal ions. Thus, U.S. Pat. No. 3,410,796 to Hull discloses a process in which boiler feed water is heated to precipitate calcium and magnesium ions as calcium carbonate, magnesium hydroxide, magnesium silicate, calcium sulfate and barium sulfate. In the Hull et al process, the raw hard water is fed to a stripper in which it is passed over a series of baffles or trays in direct countercurrent heat exchange with steam flowing upwardly from a "steam drum". Within the stripper, a portion of the steam is condensed into the water. The remainder of the steam is produced from the stripper as a high quality steam. The raw heated water then passes into the steam drum which serves as a reaction zone in which the chemical reactions are carried to completion so that a major portion of the insoluble salts are precipitated. A portion of the water in the steam drum in then continuously removed and directed into the heat exchanger tubes of a steam chest. The water is circulated through the tubes and a portion of it is converted to steam by indirect heat exchange with a heat transfer salt. The steam effluent from the heating tubes then is passed to the steam drum for circulation upwardly through the stripper column.
The use of heat to thermally induce the softening of water by the mechanisms described in the aforementioned patent to Hull, is also disclosed in Hull, "The Thermosludge Water Treating and Steam Generation Process", Journal of Petroleum Technology, December 1967 and Rintoul, "Steam from Waste Water", Pacific Oil World, July-August, 1978. As noted by the Hull and Rintoul articles, a modified stripper operation involves allowing the feed water to travel downwardly through the stripper without contact trays which were found to be subject to the formation of dolomite deposits at the top levels. Suitable softening is accomplished by the chemical reactions taking place in the 350.degree.-600.degree. F. range. As described in detail in the Hull article, where silica is present, sufficient magnesium should be present relative to the silica content to correspond to the formation of at least three moles of magnesium hydroxide for two moles of silica. The Hull and Rintoul articles also further describe the reaction conditions encountered in the softening process. They note that sodium carbonate may be added to the feed water prior to heating by direct contact with the steam to provide sufficient alkalinity for the precipitation of the magnesium as magnesium hydroxide and to provide sufficient carbonate for the precipitation of calcium carbonate. By providing these enabling conditions, the pH may increase from an initial value within the range of 7 to 8 as the feed water enters the stripper to a final value of about 11 to 12 in the boiling zone.